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22 May 2015

"THAT FIRST TURMOIL, that first bubble of foam, was the expulsion of
compressed air from the submarine’s launching tube as the torpedo
exited. The torpedo itself was 20 feet long and 20 inches in diameter;
its nose, shaped like the top of a corn silo, contained 350 pounds of
TNT and an explosive called Hexanite. Though German commanders typically
set the depth at 15 feet, this one traveled at 10 feet. It moved at
about 35 knots, or 40 miles an hour, powered by compressed air stored in
a tank toward its nose, just behind the compartment that contained the
explosives. The air rushed against the pistons in its engine, geared to
spin two propellers, one clockwise, the other counterclockwise, to keep
the torpedo from rolling and veering. The air was then exhausted into
the sea, where it bubbled to the surface. These bubbles needed a few
seconds to rise, which meant the torpedo itself was always well ahead of
the track that appeared above.As the torpedo advanced, the water
rushing past its nose turned a small propeller, which unscrewed a safety
device that prevented detonation during storage. This propeller slipped
from the nose and fell to the sea bottom, thereby exposing a triggering
mechanism that upon impact with a ship’s hull would fire a small charge
into the larger body of explosives. A gyroscope kept the torpedo on
course, adjusting for vertical and horizontal deflection.The track
lingered on the surface like a long pale scar. In maritime vernacular,
this trail of fading disturbance, whether from ship or torpedo, was
called a “dead wake.”"